1. Field of the invention
The present invention relates to a process for preparing a toner for developing an electrostatic image, used in an image forming process such as electrophotography, electrostatic recording or electrostatic printing. More particularly, the present invention is concerned with a process for producing a toner in which toner components such as a colorant and a charge control agent are uniformly dispersed in a binder resin.
2. Related Background Art
In electrophotography, it is common to employ a process comprising forming an electrostatic latent image on a photosensitive member, utilizing a photoconductive material and according to various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by various methods to obtain copies.
In general, toners are grouped into dry process toners and wet process toners. As the wet process have the problems such as evaporation of solvents, toner recovery and smell generation, and hence in recent years the dry process toners have become more prevalent.
Toner is a powder that forms an image. For the accurate image formation, toner particles must carry a number of functions, for example, chargability, transportability or fluidity, fixabiity, coloring power, storage stability and so forth. Hence, a toner is prepared in the form of a composite comprised of various kinds of materials.
The dry process toners are produced by various methods such as a pulverization process, a polymerization process and an encapsulation process. Among these methods, the pulverization process is most prevalent. When a toner is prepared by conventional pulverization method, various materials such as a binder resin that participates in the fixing of toner to transfer mediums, various colorants that give tints required for toner, as well as materials such as a charge control agent, a magnetic material, a release agent and a fluidity-providing agent, are mixed by a dry process, and thereafter melt-kneaded with shear force at a given temperature using a general-purpose kneading machine such as a kneader, an extruder or a roll mill. The kneaded product is cooled to solidify, and optionally crushed to give a crushed product material, and thereafter pulverized using a pulverizer such as a jet mill to bestow a particle diameter suitable for a toner. Thereafter, the pulverized product is classified according to the requirement, using a classifier of various types, to give desirable particle size distribution that enables the toner to exhibit satisfactory performances. Further, if necessary, functional additives such as a fluidity improving agent, a lubricant, and an abrasive can be added in a dry process to give a toner. When the toner is used as a two-component developer, the toner is mixed with a carrier of various types to prepare a two-component developer, which is used for the formation of images.
Among the factors determining the performance of the toner, there are coloring power and light transmission properties. The coloring power and light transmission properties are influenced by the amount of a colorant, and are also greatly influenced by the dispersion state of the colorant
When the toner is produced by pulverization, the dispersion states of the materials in toner particles are almost determined by the steps of mixing materials and kneading the mixed materials . As an apparatus used for the mixing of materials, a planetary-screw mixing apparatus such as a Nauta mixer as shown in FIG. 8 or a blade agitating type mixing apparatus such as a Henschel mixer as shown in FIG. 7 is usually used. A mixture obtained by such a mixing apparatus is then melt-kneaded. Although there are various types of agitators, extruders feasible for continuous kneading are usually used for mass production.
In recent years, however, as the performance of electrophotographic apparatuses such as copying machines and printers increases, the requirements for the toner performance have become much higher. It is not easy to produce toners of such high performance by the conventional process described above, in respect of, for example, fine dispersion of colorants, wetting of colorants or dispersion of other internal additives. As a result, toners unsatisfactory in dispersion or wetting of colorants tend to cause lowering of image density, unsteady performance under varying conditions, soil of developer sleeves or carriers, as well as scratches or filming on photosensitive drums.
In the kneaded product obtained in the conventional production process described above, the dispersion of the colorant in the binder resin, in particular, is unsatisfactory and hence the coloring power and light transmission properties required for toners become poor.
Now, a partly improved production process to overcome these problems is proposed, comprising a first kneading step and a second kneading step as shown in the prior art in FIG. 10. More specifically, in the first step a resin containing a colorant in a higher concentration than the final product is kneaded, and then in the second kneading step, other additives such as a binder resin and a charge control agent are further added and mixed, and the mixture is kneaded using a kneading machine such as a kneader or an extruder to give a kneaded product. A mixer used in the step of mixing materials for the first kneading step, however, is a tumbling mixer such as a V-type blender or a double-cone mixer, or a high-speed agitating mixer (FIG. 7) such as a Henschel mixer, which cannot achieve micrdispersion of materials. Hence in many instances no good dispersion or wetting of the materials can be obtained even if conditions for mixing and kneading are carefully managed. Moreover, although smaller particle size of materials may provide better wetting in principle, in practice, the agglomeration of material particles becomes too strong to achieve sufficient dispersion with ease in the course of premixing. In addition, finer particles tend to contain more air making sufficient kneading and dispersion difficult.
A typical kneading machine used in the first kneading step is a three-roll mill (FIG. 11), in which the kneading is batch-wise operation and an operator must constantly take care of the kneading process, delicately changing conditions such as kneading temperature, roll rotational speed, roll clearance, roll rotational ratio and pass frequency (frequency of treatment) to make the dispersion of colorants in the kneaded product as uniform as possible. Hence, it requires great skill. In addition, batch treatment should be repeated at least twice manually, and preferably four times or more. Thus, considering the structural safety of the machine, further improvement is required for this production process.